Thermal switches are used in a variety of applications where it is desirable to activate and/or deactivate equipment as a function of sensed temperature. Such applications may include: rocket motors and thrusters, battery charge rate control, temperature control for fuel systems, environmental controls, overheat protection as well as many others. In several thermal switch applications, it is desirable to know when the switch has been activated. For example, it is desirable to know when the switch is part of a safety system or is part of a control system used to protect delicate instrumentation. Often, there is no way of knowing that the switch has been tripped.
One application for thermal switches that clearly illustrates the disadvantages of prior art devices is duct leak overheat detection systems. The duct leak overheat detection system is part of the airplane deicing system. In this type of deicing system, hot air is forced pneumatically through a tube along the leading edge of the wing. Thermal switches located along this duct, indicate overheating, which could otherwise lead to fires and other system failures. When a thermal switch is tripped, a light illuminates in the cockpit indicating a xe2x80x9crightxe2x80x9d or xe2x80x9cleftxe2x80x9d wing overheat condition. If, after shutting the system down on the appropriate wing, the switch does not reset, the airplane must divert to an emergency landing. Upon landing, the airplane maintenance personnel have no way of knowing which particular switch has been activated, because there exist multiple thermal switches linked to a particular cockpit light. The existing airplane systems have only provided the crew with an indication of the particular wing semispan along which a thermal switch was tripped. If the switch has reset, there is no indication to the maintenance personnel that it was tripped by the overheat condition. This dearth of information requires the crew to physically access and inspect the entire system along the appropriate wing semispan. Even in applications where only one temperature probe indicated an alarm temperature in-flight, extensive and expensive troubleshooting is sometimes necessary. For example, an airborne alert from a temperature probe in aircraft turbine bleed air ductwork may require engine run-up and monitoring on the ground to determine whether the probe and/or the bleed air system is faulty.
The present invention provides a ready indication that the thermal switch has experienced temperatures that triggered operation of the device. According to one aspect of the present invention, a temperature sensitive material in the form of a label or decal is affixed to the outside of the thermal switch. The temperature sensitive label provides a permanent record of the temperature limits that the switch has been exposed to. The temperature sensitive label changes colors when the thermal switch is exposed to its predetermined temperature limit. The changed color provides a quick and clear indication of an event that caused switch closure. The color indication on the outside surface of the switch also provides a visual indication that is easy to acquire and inspect without the need to have physical access to the switch itself.
According to another aspect of the present invention, the thermal switch of the present invention is especially suited for use as an overheat sensor in airplane deicing systems or in aircraft turbine engine bleed air ductwork. The maintenance crew can quickly locate and identify an activated switch according to decal that changed color.